Radio Frequency Switching Circuit with Distributed Switches

ABSTRACT

An RF switching device having distributed shunt switches distributed along transmission lines to improve RF bandwidth as well as the signal isolation of the device. The shunt switches may be physically positioned on both sides of the transmission lines to keep an integrated circuit (IC) design essentially symmetrical so as to provide predictable and reliable operational characteristics. Some embodiments include stacked FET shunt switches and series switches to tolerate high voltages. In some embodiments, the gate resistor for each FET shunt switch is divided into two or more portions.

CROSS REFERENCE TO RELATED APPLICATION—CLAIM OF PRIORITY

This application is a continuation-in-part of commonly owned andco-pending U.S. patent application Ser. No. 14/610,588 filed Jan. 30,2015, entitled “Radio Frequency Switching Circuit with DistributedSwitches”, the disclosure of which is incorporated herein by referencein its entirety.

BACKGROUND

1. Technical Field

This invention generally relates to electronic signal switching devices,and more specifically to electronic radio frequency signal switchingdevices.

2. Background

Electronic signal switches are used in a wide variety of applications.One type of signal switch in common use is a field effect transistor(FET) that is actively controlled through a gate terminal to block orpass an electrical signal connected in series with source and drainterminals of the FET (in another mode of operation, a FET also may beused to modulate an electrical signal in response to a varying signal onthe gate terminal).

Field effect transistors may be fabricated in various technologies(e.g., standard bulk silicon, silicon-on-insulator, silicon-on-sapphire,GaN HEMT, GaAs pHEMT, and MESFET processes) and are commonly representedin schematic diagrams as an idealized device. However, in manyapplications, particularly in radio frequency (RF) circuits, thestructure and materials of a FET switch may have significant effects onits own operation (e.g., with respect to bandwidth, isolation, and powerhandling) and the presence of a FET switch may have significant effectson other components in a circuit. Such effects arise in part because a“CLOSED”/“ON” (low impedance) FET has a non-zero resistance, and an“OPEN”/“OFF” (high impedance) FET behaves as a capacitor due toparasitic capacitances arising from the proximity of varioussemiconductor structures, particularly within the close confines of anintegrated circuit (IC). Large signal behaviors affecting power handlingmay also arise from other characteristics of a FET, such as avalanchebreakdown, current leakage, accumulated charges, etc. Accordingly, theactual in-circuit behavior of a FET must be taken into account whendesigning FET based circuitry.

One use of FET switches is within RF frequency signal switching devices.For example, FIG. 1A is a schematic diagram of a prior art 3-portreflective signal switching device 100 for selectively coupling one oftwo terminal ports 102A, 102B (shown series connected to respectiveexternal loads RF1, RF2) to a common port 104 (shown series connected toan external load RFC). Accordingly, the signal switching device 100 maybe regarded as a single-pole, double-throw (SPDT) switch. In otherconfigurations, more than two terminal ports (a 1×N switch) and morethan one common port may be included (an M×N switch). Between the commonport 104 and each terminal port 102A, 102B are respective FET seriesswitches 106A, 106B; the FET series switches 106A, 106B may vary insize, for example, to accommodate different power levels. Between eachterminal port 102A, 102B and its respective series switch 106A, 106B arerespective FET shunt switches 108A, 108B, coupled to circuit ground.Such a switching device 100 may be used, for example, to selectivelycouple RF signals between two antennas respectively connected to theterminal ports 102A, 102B and transmit and/or receive circuitryconnected to the common port 104. For RF signals, each load/sourceimpedance RF1, RF2, RFC would typically have a nominal impedance of 50ohms by convention.

In operation, when terminal port 102A is to be coupled to the commonport 104, series switch 106A is set to a low impedance ON state by meansof control circuitry (not shown) coupled to the gate of the FET seriesswitch 106A. Concurrently, shunt switch 108A is set to a high impedanceOFF state. In this state, signals can pass between terminal port 102Aand the common port 104.

For the other terminal port 102B, the series switch 106B is set to ahigh impedance OFF state to decouple the terminal port 102B from thecommon port 104, and the corresponding shunt switch 108B is set to a lowimpedance ON state. One purpose of setting the shunt switch 108B toON—thus coupling the associated terminal port 102B to circuit ground—isto improve the isolation of the associated terminal port 102B (andcoupled circuit elements, such as antennas) through the correspondingseries switch 106B. For switching devices with more than two terminalports, the series switch and shunt switch settings for the “unused”(decoupled) terminal port to common port signal paths typically would beset to similar states.

FIG. 1B is a diagram showing an equivalent circuit model of the priorart 3-port signal switching device of FIG. 1A. Shown is a circuitconfiguration 120 in which terminal port 102A has been coupled to thecommon port 104; accordingly, series switch 106A and shunt switch 108Bare set to a low impedance ON state, while series switch 106B and shuntswitch 108A are set to a high impedance OFF state. In thisconfiguration, series switch 106A is modeled as a resistor 126A having aresistance value of Ron (i.e., the CLOSED or ON state resistance of aFET), shunt switch 108A is modeled as a capacitor 128A having acapacitance of Cshunt (i.e., the OPEN or OFF state capacitance of aFET), series switch 106B is modeled as a capacitor 126B having acapacitance of Coff, and shunt switch 108B is modeled as a resistor 128Bhaving a resistance value of Rshunt. As in FIG. 1A, with the illustratedcircuit configuration, signals can pass between terminal port 102A andthe common port 104.

FIG. 1C is a diagram showing a simplified equivalent circuit model 130corresponding to the circuit configuration 120 shown in FIG. 1B. Seriesswitch 106B (modeled as a capacitor 126B in FIG. 1B) is OFF. Thecorresponding shunt switch 108B (modeled as a resistor 128B in FIG. 1B)is ON, thus having a very low impedance and coupling terminal port 102Bto circuit ground. Since Rshunt has a very low impedance, the resistorequivalent 128B in FIG. 1B may be more simply modeled as a conductor(short) to circuit ground and is thus shown in dotted-line resistorform. Therefore, the two equivalent circuit elements 126B, 128B of FIG.1B may be modeled as a single capacitor 126B′ having a capacitance ofCoff. Similarly, since series switch 106A (modeled as a resistor 126A inFIG. 1B) is ON and Ron is a very low impedance, series switch 106A maybe more simply modeled as a conductor. Accordingly, the resistorequivalent 126A in FIG. 1B is shown in dotted-line resistor form,leaving OFF shunt switch 108A (modeled as a capacitor 128A with acapacitance of C shunt) connected in parallel with the external loadRF1. As in FIG. 1A and FIG. 1B, with the illustrated circuitconfiguration, signals can pass between terminal port 102A and thecommon port 104, as shown by dotted line signal path 132.

The simplified equivalent circuit model 130 can be used to evaluate theinsertion loss (IL) bandwidth of the circuit model 130. In this example,the 3 dB IL bandwidth is proportional to 1/(Rport*(Coff+Cshunt)) [whereRport is the load resistance at the RF1 and RFC ports], which istypically limited to below 13 GHz in current silicon IC technology.

The bandwidth of conventional radio frequency switching devices of thetype shown in FIGS. 1A, 1B, and 1C is limited by the parasiticcapacitance from the Cshunt equivalent components. This invention invarious embodiments addresses this limitation to improve the bandwidthof RF switching devices as well as the signal isolation and powerhandling of such switching devices.

SUMMARY OF THE INVENTION

Embodiments of the invention use distributed shunt switches distributedalong transmission lines (or may include other inductive impedancecompensating components) to improve RF bandwidth with respect toinsertion loss, and to improve isolation. In addition, the shuntswitches may be physically positioned on both sides of the transmissionlines to keep an integrated circuit (IC) design essentially symmetricalso as to provide predictable and reliable operational characteristics.Some embodiments include stacked FET shunt switches and series switchesto tolerate high voltages. In some embodiments, the gate resistor foreach FET shunt switch is divided into two or more portions to save ICarea near the transmission lines, or to optimize a performanceparameter, such as power handling, isolation, or low frequency behavior.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a prior art 3-port reflective signalswitching device for selectively coupling one of two terminal ports to acommon port.

FIG. 1B is a diagram showing an equivalent circuit model of the priorart 3-port signal switching device of FIG. 1A.

FIG. 1C is a diagram showing a simplified equivalent circuit modelcorresponding to the circuit configuration shown in FIG. 1B.

FIG. 2A is a schematic diagram of a 3-port signal switching device forselectively coupling one of two terminal ports to a common port inaccordance with the teachings of this disclosure.

FIG. 2B is a schematic representation of an elemental length of atransmission line.

FIG. 2C is a diagram showing an equivalent circuit model of the 3-portsignal switching device of FIG. 2A.

FIG. 3 is a graph showing simulation results of three variations of aswitching device in accordance with FIG. 2A.

FIG. 4A is a schematic diagram of a circuit architecture havingdistributed stacked shunt switches as well as distributed gateresistors.

FIG. 4B is a schematic diagram of a circuit architecture having lumpedstacked shunt switches.

FIG. 5 is a schematic diagram of a circuit architecture having stackedseries switches.

FIG. 6 is a diagram of a conceptual circuit layout of an RF switchingcircuit with distributed stacked switches for both the shunt and seriesswitch components.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

The bandwidth of conventional radio frequency (RF) switching devices ofthe type shown in FIGS. 1A, 1B, and 1C is limited by the parasiticcapacitance from the Cshunt equivalent components. Embodiments of theinvention use distributed shunt switches distributed along transmissionlines (or may include other inductive impedance compensating components)to improve RF bandwidth with respect to insertion loss, and to improveisolation. In addition, the shunt switches may be physically positionedon both sides of the transmission lines to keep an integrated circuit(IC) design essentially symmetrical so as to provide predictable andreliable operational characteristics. Some embodiments include stackedFET shunt switches and series switches to tolerate high voltages. Insome embodiments, the gate resistor for each FET shunt switch is dividedinto two or more portions to save IC area near the transmission lines,or to optimize a performance parameter, such as power handling,isolation, or low frequency behavior.

Distributed Shunt Switches

FIG. 2A is a schematic diagram of a 3-port signal switching device 200for selectively coupling one of two terminal ports 102A, 102B (shownseries connected to respective external loads RF1, RF2) to a common port104 (shown series connected to an external load RFC) in accordance withthe teachings of this disclosure. Accordingly, the illustrated signalswitching device 200 may be regarded as a single-pole, double-throw(SPDT) switch.

Between the common port 104 and each terminal port 102A, 102B arerespective primary isolation FET series switches 201A, 201B that operatein essentially the same fashion as the corresponding series switches106A, 106B in FIG. 1A. In other configurations, only one terminal portmay be used (e.g., only terminal port 102A in a single-pole,single-throw switch), more than two terminal ports may be included(e.g., a 1×N switch), and more than one common port may be included(e.g., an M×N matrix switch). When three or more terminal ports areincluded (e.g., 102A, 102B, and a similar third port not shown), theswitching device 200 may be utilized as a transfer switch, allowing asignal to be communicated from any one port to any other port whileisolating unused ports from the signal path (the common port 104 is notused in such a case unless an isolation serial switch to the signal pathis interposed). Further, the FET series switches 201A, 201B may be ofdifferent sizes in some embodiments. The illustrated embodiment may beadvantageously embodied on a silicon-on-insulator (SOI) integratedcircuit (IC) substrate.

An important aspect of the disclosed embodiments is that inductivetuning components are included to compensate for the OFF statecapacitance Cshunt of the shunt switch units 204 described below. Oneway to provide such inductive tuning components is to use a transmissionline that includes at least one series inductive component coupled to atleast one shunt capacitive component. In the embodiment illustrated inFIG. 2A, each of the FET series switches 201A, 201B is coupled to acorresponding transmission line 202A, 202B that can be modeled as aplurality of series-coupled inductive tuning components 203 (depicted asrectangular symbols in this example). Each transmission line 202A, 202Bmay be implemented, for example, as microstrips or coplanar waveguides.For RF switching devices, the transmission lines 202A, 202B wouldtypically be tuned to have a nominal impedance of 50 ohms by convention.The individual shunt switches 108A, 108B shown in FIG. 1A have each beenreplaced by sets of n (where n≧1) parallel FET shunt switch units 204.The shunt switch units 204 may be reduced in size compared to aconventional single shunt switch 108A, 108B.

FIG. 2B is a schematic representation of an elemental length of atransmission line, where Rdx, Ldx, Gdx, and Cdx, are respectively, theper unit length resistance, inductance, conductance, and capacitance ofthe line. The impedance Zo of such a transmission line is Zo=√(Ldx/Cdx).The OFF state capacitance Cshunt of the shunt switches units 204 is inparallel with Cdx. To achieve compensation of Cshunt of the shuntswitches units 204, Ldx can be increased, or Cdx can be decreased, orboth, with respect to each other, so that √(Ldx/(Cdx+Cshunt))=Zo(commonly specified as 50 ohms by convention).

In the illustrated embodiment, the conduction (source-drain) channel ofeach FET shunt switch unit 204 is coupled to circuit ground and betweena corresponding pair of inductive tuning components 203, thereby formingan elemental length of a transmission line 206, examples of which areshown bounded by dotted boxes. In some embodiments, an inductive tuningcomponent 203 may be shared between adjacent shunt switch units 204,thus constituting part of two elemental lengths of a transmission line.However, for purposes of circuit analysis, it may be easier to model ashared inductive tuning component 203 as being “split” between adjacentshunt switch units 204.

As more fully explained below, the primary series switches 201A, 201Band the shunt switch units 204 may be replaced by multipleseries-coupled FET switches to tolerate higher voltages than a singleFET switch. Such “stacking” of FET switches helps decrease the effectiveCshunt while permitting higher power handling.

Additional supplemental inductive tuning components 207 (also labeledL_(a) and L_(b)) may be added at either end or both ends of thetransmission lines 202A, 202B to enable fine tuning of parasiticsunrelated to the transmission lines 202A, 202B, such as the seriesswitch device parasitic capacitances and pad capacitance for I/Ointerconnects. The values for the supplemental inductive tuningcomponents 207 (L_(a), L_(b)) of one transmission line may be the sameor different with respect to each other, and with respect to thesupplemental inductive tuning components 207 (L_(a), L_(b)) of othertransmissions lines. In addition, in some applications, it may be usefulto insert an additional secondary isolation series FET switch in serieswith the transmission line signal path at each node marked “X” in FIG.2A and FIG. 2C, making the circuit appear more symmetrical electrically.By opening the primary and secondary series FET switches at both ends ofa transmission line (e.g., 202B) associated with an unused terminal port(e.g., 102B), that transmission line will be fully isolated from anycircuitry attached to such ends. The secondary isolation series FETswitches may be the same size as the primary isolation series switches201A, 201B, but equal sizing is not required to still provide benefit.For example, modeling has shown that there may be a benefit to havingdifferent sizes for the secondary isolation series FET switches relativeto the primary isolation series FET switches. In general, addingsecondary isolation series FET switches would most frequently apply toswitching devices having two or more terminal ports, but may also beapplied to switching devices having a single terminal port (e.g., a SPSTswitch) when there may be some aspect of the external circuit elementbeing switched where having more isolation on both sides of thetransmission line signal path is helpful (e.g., when using an SPSTembodiment in a shunt configuration to circuit ground).

In operation, when terminal port 102A is to be coupled to the commonport 104, series switch 201A is set to a low impedance ON state by meansof control circuitry (not shown) coupled to the gate of the FET seriesswitch 201A. Concurrently, the set of n shunt switch units 204 coupledto transmission line 202A is set to a high impedance OFF state. In thisstate, signals can pass between terminal port 102A and the common port104 along transmission line 202A.

For the other terminal port 102B in this example, the series switch 201Bis set to a high impedance OFF state to decouple transmission line 202Band the terminal port 102B from the common port 104, and the set of ncorresponding shunt switch units 204 coupled to transmission line 202Bis set to a low impedance ON state, thus coupling the associatedterminal port 102B to circuit ground.

FIG. 2C is a diagram showing an equivalent circuit model of the 3-portsignal switching device of FIG. 2A. Shown is a circuit configuration 250in which terminal port 102A has been coupled to the common port 104, asdescribed with respect to FIG. 2A. In this configuration, series switch201A is modeled as a resistor 210 having a resistance value of Ron, andseries switch 201B is modeled as a capacitor 212 having a capacitance ofCoff. The shunt switch units 204 coupled to transmission line 202A areeach shown modeled as capacitances 220 with a capacitance of Cshunt/n.The shunt switch units 204 coupled to transmission line 202B are eachshown modeled as resistances 222 with a resistance of Rshunt*n.

As in FIG. 2A, with the illustrated circuit configuration 250, signalscan pass between terminal port 102A and the common port 104. For RFsignals, each load RF1, RF2, RFC would typically have a nominalimpedance of 50 ohms by convention. Each of the inductive tuningcomponents 203, 207 in FIG. 2A has a corresponding inductance, L_(a),L_(b), or L₁, as shown in FIG. 2C and described below. Values for theinductive tuning components 203 may be selected to achieve compensationof Cshunt of the shunt switches units 204 as described above. Values forthe supplemental inductive tuning components 207 (L_(a), L_(b)) may beselected to achieve compensation of Coff and the impedance (Zoff_(a),described below) of the OFF signal path, and for parasitics associatedwith signal interconnections.

Benefits of the embodiment illustrated in FIG. 2A and FIG. 2C include(1) tuning out the effect of Coff; (2) tuning out the effect of C shunt;and (3) improving isolation of OFF paths.

(1) Tuning out the effect of Coff

For the configuration shown in FIG. 2A, the impedance Zoff of the OFFpath (i.e., all of the elements from series switch 201B through loadRF2, as indicated by the dotted “Zoff” line in FIG. 2C) is given by thefollowing formula:

Zoff=jω(L_(a) +L1)+1/jωCoff+Zoff_(a)   [Eq. 1]

where Zoff_(a) comprises the impedance of the OFF path after the firstL₁ inductive tuning component through load RF2 (as indicated by thedotted “Zoff_(a)” line in FIG. 2C) and whose value approaches n*Rshuntat higher frequencies.

The resonant frequency of the Zoff impedance is

$\frac{1}{2\; \pi \sqrt{( {L_{a} + L_{1}} ){Coff}}}.$

When Zoff is below its resonant frequency

$( {{i.e.},{{j\; {w( {L_{a} + L_{1}} )}} < {\frac{1}{j\; \omega \; {coff}}}}} ),$

achieved by selection of the values for the inductive tuning components203 for a particular application, then the loading effect of the Coffcapacitance on the ON path (i.e., all of the elements from series switch201A through load RF1) is appreciably reduced, thus improving thebandwidth of the switching device 200 compared with conventionaldesigns. This characteristic can be used to improve the design trade-offbetween bandwidth, insertion loss, and isolation for all such switchingdevices.

(2) Tuning Out the Effect of Cshunt

For the configuration shown in FIG. 2A, with respect to the ON path, theimpedance Zon for the ON path (as indicated by the dotted “Zon” line) isgiven by the following formula:

$\begin{matrix}{{Zon} = \sqrt{\frac{2*L_{1}}{{Chsunt}/n}}} & \lbrack {{Eq}.\mspace{14mu} 2} \rbrack\end{matrix}$

The cutoff frequency (half power point), f_(c), is given by thefollowing formula:

$\begin{matrix}{f_{c} = \frac{1}{2\; \pi \sqrt{2*L_{1}*\frac{Cshunt}{n}}}} & \lbrack {{Eq}.\mspace{14mu} 3} \rbrack\end{matrix}$

Accordingly, the half power point (3 dB) bandwidth of Zon is related toL₁, n, and Cshunt, and can be adjusted by adding additional tuningnetwork stages 206 (i.e., increasing n). The corresponding value of L₁is then determined by Eq. 2 to maintain a constant Zon. As deduced fromEq. 2, as n is increased, the corresponding value of Li is decreasedproportional to 1/n. In particular, the higher the number n of tuningnetworks 206, the higher the cutoff frequency. For example, FIG. 3 is agraph showing simulation results of three variations of a switchingdevice in accordance with FIG. 2A, with n=4, 5, or 6 while keeping thetotal transmission line 202A length and the total Cshunt capacitance thesame. Utilizing a more conservative power point before insertion lossbegins to significantly decline, the 1.5 dB bandwidth (indicated bycorresponding markers m1, m2, and m3) for the three circuit variationsimproved from 21.5 GHz (n=4, see line 302) to 25.4 GHz (n=5, see line304) to 29.9 GHz (n=6, see line 306) as n increased.

Further, working with equations Eq. 2 and Eq. 3, the value of Cshunt canbe expressed in terms of the desired Zon, f_(c), and number of networksn as follows:

$\begin{matrix}{{Cshunt} = \frac{n}{2\; \pi \; f_{c}*{Zon}}} & \lbrack {{Eq}.\mspace{14mu} 4} \rbrack\end{matrix}$

Therefore, the maximum Cshunt can be calculated for a set of targetedparameters. As an example, for a Zon of 50 Ohms, a cutoff frequency of60 GHz, and n=6 for the number of tuning networks 206, results inCshunt=318 fF, Cshunt/n=53 fF, and L₁=66 pH.

(3) Improving Isolation of OFF Paths

For the configuration shown in FIG. 2A, as noted above, the higher thenumber n of shunt switch units 204 and corresponding inductive tuningcomponents 203 (i.e., tuning networks 206), the higher the cutofffrequency. In addition, as n increases, the higher the number of LR lowpass filter stages there are in the OFF path, and accordingly theisolation of the OFF path (from RFC to RF2) is improved compared toconventional designs.

Stacked Switch Structures

As mentioned above, each of the shunt switch units 204 and the seriesswitches 201A, 201B may be replaced by multiple series-coupled FETswitches. This type of “stacked” architecture allows a circuit totolerate higher voltages than a single FET switch. For example, FIG. 4Ais a schematic diagram of a circuit architecture having distributedstacked shunt switches as well as distributed gate resistors (the gateresistor aspect is discussed below). In this example, each of the nshunt switch units 204 of FIG. 2A has been replaced by a series-coupledstack of m FET switches 402, where m≧2.

For some embodiments that may not require distributed shunt switches, alumped design with stacked shunt switches may be used. For example, FIG.4B is a schematic diagram of a circuit architecture having lumpedstacked shunt switches. In this example, the n shunt switch units 204 ofFIG. 2A has been replaced by a single series-coupled stack of m FETswitches 404.

The series switches 201A, 201B shown in FIG. 2A may also be implementedas stacked switches. For example, FIG. 5 is a schematic diagram of acircuit architecture 500 having stacked series switches. One or more ofthe series switches 201A, 201B shown in FIG. 2A would be replaced by twoor more FET switches 502 configured as a series-coupled stack.

Symmetrical Layout

The switching device architecture shown in FIG. 2A can be advantageouslycombined with the stacked switch circuits shown in FIG. 4A, FIG. 4B,and/or FIG. 5 to provide a distributed stacked FET-switch basedswitching device that provides for approximately even electromagneticfield distribution around the transmission lines 202A, 202B, andprovides for a better ground return.

For example, FIG. 6 is a diagram of a conceptual circuit layout of an RFswitching circuit 600 with distributed switches (which may be stackedswitches) for both the shunt and series switch components. In theillustrated embodiment, two terminal ports 102A, 102B (shown seriesconnected to respective external loads RF1, RF2) are connectable to acommon port 104 (shown series connected to an external load RFC) throughcorresponding transmission lines 202A, 202B and series switches 602A,602B. The series switches 602A, 602B may be single FET switches as shownin FIG. 2A, or a stack of FET switches as shown in FIG. 5. As in FIG. 2Aand FIG. 2C, the transmission lines 202A, 202B each include a pluralityof inductive tuning components 203, and, optionally, supplementalinductive tuning components 207 (not shown).

Coupled to the transmission lines 202A, 202B are sets of n shuntswitches 604, each of which may be configured as shown in FIG. 2A(distributed), FIG. 4A (stacked distributed), or FIG. 4B (lumpeddistributed); in each configuration, there is an internal connection tocircuit ground (not shown in FIG. 6). As discussed above, aconfiguration with n shunt switch units alone or in conjunction withcontrol of the inductance values of the inductive tuning components 203,207 gives control over the cutoff frequency of the switching circuit600, provides the ability to tune out the effects of Cshunt and Coff,and improves isolation of OFF paths. In addition, by using a stack of mFET switches for each of the n shunt switch units, higher voltage levelscan be tolerated.

Importantly, in the configuration shown in FIG. 6, the sets of shuntswitches 604 are physically placed on both sides of the transmissionlines 202A, 202B, and the transmission lines 202A, 202B are arrayed onan IC layout in a substantially symmetrical manner. Such placement ofthe sets of shunt switches 604 provides for approximately evenelectromagnetic field distribution around the transmission lines 202A,202B and provides for a better ground return because of the multipleconnections to circuit ground; both characteristics are useful whendesigning absorptive switches. In addition, such physical distributionimproves the thermal characteristics of the switching device 600 byspacing apart the FET switches, thus reducing the areal concentration ofpower-consuming circuit elements.

As noted above, in other configurations, only one terminal port may beused (e.g., a single-pole, single-throw switch), more than two terminalports may be included (e.g., a 1×N switch), and more than one commonport may be included (e.g., an M×N matrix switch). Accordingly, fewer oradditional transmission lines may be arrayed on an IC layout in asubstantially symmetrical manner as needed to accommodate fewer oradditional ports, with associated sets of shunt switches 604 physicallyplaced on both sides of the added transmission lines.

Gate Resistance Area Reduction

In general, FET switches require a gate resistor to limit theinstantaneous current that is drawn when the FET is turned on, tocontrol the switch ON and OFF times, and in general to maintainelectromagnetic integrity. In conventional IC FET designs, a gateresistor is physically located in close proximity to the gate of thetransistor. However, when implementing a distributed shunt switch of thetype shown in FIG. 4A, each FET shunt switch unit 204 is n times smallerthan in a lumped design. Accordingly, to maintain the same low frequencycharacteristics, the gate resistor value for each FET shunt switch unit204 must be n times larger than in a lumped design. Further, each FETshunt switch unit 204 may include m FET switches 402. In such aconfiguration, since there are m stacked elements per shunt switch unit204, the total size for all of the gate resistors is m*n times bigger inarea than in a lumped design.

To reduce the total size of the needed gate resistance, in someembodiments a FET gate resistor can be split into two sections.Referring again to FIG. 4A, each of the FET switches 402 includes asmall primary resistance R1 (e.g., about from 1,000 to 1,000,000 ohms)that is placed in close proximity to the gate of each FET to take careof needed electromagnetic integrity. A larger secondary resistance R2(e.g., about from 10,000 to 10,000,000 ohms) is then placed on a commonpath series coupled to multiple instances of the small primaryresistance R1 (e.g., each of the n FET shunt switch units 204) tomaintain desired low frequency characteristics, but may be physicallylocated away from close proximity to the small primary resistances R1(and hence from the gate of each FET). The values for R1 and R2 are setsuch that R1/n+R2=R, where R is the total gate resistance needed for aparticular circuit design. Such values may be empirically determined byexperiment or simulation for frequencies of interest. Because the largersecondary resistances R2 are shared over a number of FET switches, thetotal area needed for integrated circuit fabrication will be reduced inscale by the ratio of R1 to R2. As should be apparent, each primaryresistance R1 and secondary resistance R2 may comprise two or moreactual resistive elements.

Methods

Another aspect of the invention includes a method for configuring aradio frequency switching device, including the steps of:

STEP 1: providing at least one common port;

STEP 2: providing at least one field effect transistor (FET) seriesswitch, each coupled to at least one common port;

STEP 3: providing at least one transmission line, each coupled to arespective FET series switch, each transmission line including at leastone series-coupled inductive tuning component;

STEP 4: providing at least one terminal port, each coupled to arespective transmission line; and

STEP 5: providing, for each transmission line, at least one FET shuntswitch unit coupled to circuit ground and to such transmission line in atuning network configuration.

A further aspect of the invention includes a method for configuring aradio frequency switching device, including the steps of:

STEP 1: providing at least one common port;

STEP 2: coupling at least one field effect transistor (FET) seriesswitch to at least one common port;

STEP 3: coupling at least one transmission line to a respective FETseries switch, each transmission line including at least oneseries-coupled inductive tuning component;

STEP 4: coupling at least one terminal port to a transmission line; and

STEP 5: coupling at least one FET shunt switch unit to circuit groundand to each such transmission line in a tuning network configuration.

The described method can be extended to include physically positioningpairs of the FET shunt switch units on both sides of each of the atleast one transmission line; arraying the at least one transmission lineon an integrated circuit layout in a substantially symmetrical manner;configuring at least one FET shunt unit as a series-coupled stack of FETswitches; configuring at least one FET series switch as a series-coupledstack of FET switches; coupling at least one primary resistor to a gateof each FET in the FET shunt unit in close proximity to such gate, andproviding a plurality of secondary resistors each series coupled to theprimary resistors of two or more FETs but located farther away from thegate of each such FET than the primary resistors coupled to each suchgate; coupling at least one secondary isolation FET series switchbetween a respective one of the at least one transmission line and arespective one of the at least one terminal port; and fabricating thedescribed circuitry as an integrated circuit.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Thus, selection of suitable component valuesare a matter of design choice unless otherwise noted. The switching andpassive elements may be implemented in any suitable integrated circuit(IC) technology, including but not limited to MOSFET and IGFETstructures. Integrated circuit embodiments may be fabricated using anysuitable substrates and processes, including but not limited to standardbulk silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS),GaAs pHEMT, and MESFET processes. Voltage levels may be adjusted orvoltage polarities reversed depending on a particular specificationand/or implementing technology (e.g., NMOS, PMOS, or CMOS). Componentvoltage, current, and power handling capabilities may be adapted asneeded, for example, by adjusting device sizes, “stacking” components totolerate greater voltages (including as described above), and/or usingmultiple components in parallel to tolerate greater currents. Additionalcircuit components may be added to enhance the capabilities of thedisclosed circuits and/or to provide additional functional withoutsignificantly altering the functionality of the disclosed circuits.

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. Various activitiesdescribed with respect to the methods identified above can be executedin repetitive, serial, or parallel fashion. It is to be understood thatthe foregoing description is intended to illustrate and not to limit thescope of the invention, which is defined by the scope of the followingclaims, and that other embodiments are within the scope of the claims.

What is claimed is:
 1. A radio frequency switching device including: (a)at least one common port; (b) at least one field effect transistor (FET)series switch, each coupled to at least one common port; (c) at leastone transmission line, each coupled to a respective one of the at leastone FET series switch, each transmission line including at least oneseries-coupled inductive tuning component; (d) at least one terminalport, each coupled to a respective one of the at least one transmissionline; and (e) for each transmission line, at least one FET shunt switchunit coupled to circuit ground and to such transmission line in a tuningnetwork configuration.
 2. The radio frequency switching device of claim1, wherein each transmission line is configured so as to have two sidesand pairs of the at least one FET shunt switch unit are physicallypositioned on both sides of each transmission line.
 3. The radiofrequency switching device of claim 1, wherein the at least onetransmission line is arrayed on an integrated circuit layout in asubstantially symmetrical manner.
 4. The radio frequency switchingdevice of claim 1, wherein at least one FET shunt unit includes aseries-coupled stack of FET switches.
 5. The radio frequency switchingdevice of claim 1, wherein at least one FET series switch includes aseries-coupled stack of FET switches.
 6. The radio frequency switchingdevice of claim 1, wherein each FET of the at least one FET shunt switchunit includes a primary resistor coupled to a gate of such FET in closeproximity to such gate, and further including a plurality of secondaryresistors each series coupled to the primary resistor of two or moreFETs but located farther away from the gate of each such FET than theprimary resistor coupled to each such gate.
 7. The radio frequencyswitching device of claim 1, wherein at least one transmission linefurther includes at least one coupled supplemental inductive tuningcomponent.
 8. The radio frequency switching device of claim 1, whereineach FET shunt switch unit has an OFF state capacitance, and the tuningnetwork configuration compensates for the OFF state capacitance.
 9. Theradio frequency switching device of claim 1, further including at leastone secondary isolation FET series switch, each coupled between arespective one of the at least one transmission line and a respectiveone of the at least one terminal port.
 10. A radio frequency switchingdevice including: (a) at least one common port; (b) a plurality of fieldeffect transistor (FET) series switches, each coupled to at least onecommon port; (c) a plurality of transmission lines, each coupled to arespective one of the plurality of FET series switches, eachtransmission line including one or more compensating inductanceelements; (d) a plurality of terminal ports, each coupled to arespective one of the plurality of transmission lines; and (e) for eachtransmission line, at least one FET shunt switch unit, wherein each FETshunt unit is coupled to circuit ground and to such one or morecompensating inductance elements in an associated tuning networkconfiguration, wherein each FET shunt switch unit has an OFF statecapacitance and the associated tuning network configuration compensatesfor such OFF state capacitance.
 11. A method for configuring a radiofrequency switching device, including the steps of: (a) providing atleast one common port; (b) coupling at least one field effect transistor(FET) series switch to at least one common port; (c) coupling at leastone transmission lines to a respective one of the at least one FETseries switch, each transmission line including at least oneseries-coupled inductive tuning component; (d) coupling at least oneterminal port to a respective one of the at least one transmission line;and (e) coupling at least one FET shunt switch unit to circuit groundand to each such transmission line in a tuning network configuration.12. The method of claim 11, further including configuring eachtransmission line so as to have two sides and positioning pairs of theat least one FET shunt switch unit on both sides of each transmissionline.
 13. The method of claim 11, further including arraying the atleast one transmission line on an integrated circuit layout in asubstantially symmetrical manner.
 14. The method of claim 11, wherein atleast one FET shunt unit includes a series-coupled stack of FETswitches.
 15. The method of claim 11, wherein at least one FET seriesswitch includes a series-coupled stack of FET switches.
 16. The methodof claim 11, wherein each FET of the at least one FET shunt switch unitincludes a primary resistor coupled to a gate of such FET in closeproximity to such gate, and further including coupling a plurality ofsecondary resistors in series to the primary resistor of two or moreFETs but located farther away from the gate of each such FET than theprimary resistor coupled to each such gate.
 17. The method of claim 11,further including coupling at least one supplemental inductive tuningcomponent to at least one transmission line.
 18. The method of claim 11,wherein each FET shunt switch unit has an OFF state capacitance, and thetuning network configuration compensates for the OFF state capacitance.19. The method of claim 11, further including coupling at least onesecondary isolation FET series switch between a respective one of the atleast one transmission line and a respective one of the at least oneterminal port.
 20. A method for configuring a radio frequency switchingdevice, including the steps of: (a) providing at least one common port;(b) coupling a plurality of field effect transistor (FET) seriesswitches to at least one common port; (c) coupling a plurality oftransmission lines to a respective one of the plurality of FET seriesswitches, each transmission line including one or more compensatinginductance elements; (d) coupling a plurality of terminal ports to arespective one of the plurality of transmission lines; and (e) couplingat least one FET shunt switch unit to each transmission line, whereineach FET shunt switch unit is coupled to circuit ground and to such oneor more compensating inductance elements in an associated tuning networkconfiguration, wherein each FET shunt switch unit has an OFF statecapacitance and the associated tuning network configuration compensatesfor such OFF state capacitance.